1. Field of the Invention
The invention relates to a process and apparatus for offset printing, and more particularly to a process and apparatus for dry offset printing on objects with a non-absorbent surface.
2. Description of Related Art
What is known as "offset" printing is generally based on the use of a plate, of which a "hydrophobic" part is inked, as opposed to a "hydrophilic" part that is moistened with water, which in turn repels the ink.
The technique known as "dry offset" printing is accordingly more applicable in certain aspects to typography than to offset printing.
In particular, dry offset printing generally uses inks in the tint of the decoration to be made and makes no, or only slight, use of the four color process with which any decoration can be made from what are known as "primary" inks (yellow, magenta, cyan).
The principle of dry offset printing is illustrated in FIG. 1. Generally, a conventional apparatus includes:
A plurality of inking blocks (one inking block BEi per color or corresponding ink Ei), each one inking a relief plate (CRi) mounted on a drum, corresponding to one color of the final pattern to be reproduced on an object.
Each inking block BEi includes an inker and a succession of transfer rollers intended to obtain a regular and constant thickness of ink on each inked plate.
A central blanket mounted on the drum, each of the relief plates (CRi) being in contact with the rubber blanket (BL) in such a way that each decorative element on each of the plates (CRi) is transferred to the blanket, and that upon each rotation of the blanket one complete decoration is formed on the blanket.
Means making it possible to put the central blanket into contact with a drum carrying the object to be decorated OD, and means for transferring the objects before and after decoration, the object being either a sheet mounted on the drum, or more often an object with a cylindrical or frustoconical surface of revolution.
In operation, heating of the ink is observed from the inker to the relief plate, where the ink typically reaches a temperature of 40.degree. C. Corresponding to this heating of the ink is a related lessening in tackiness, that is, in the inclination an ink has of being transferred from one ink transfer roller to the next roller.
Tackiness is an essential characteristic of a dry offset ink and is measured with the aid of a tackometer, which in the case of the apparatus used by the present applicant functions in accordance with the drawing in FIG. 2.
In using a dry offset printing apparatus according to the prior art, applicant has observed several types of problems
Difficulties in obtaining saturated tints, particularly in the case where bar codes are produced with a black ink, which is a prohibitive defect, since it makes reading the bar code difficult (several attempts being necessary to successfully read the bar code), or even impossible: the overly liquid ink has been unable to accumulate thickly enough.
Second, difficulties in obtaining tints of constant intensity over the course of production, requiring permanent modification of the settings of the machine and local pressure on the inking blade, which may lead to the development of undesirable residues in the inker.
Finally, the appearance of powdered deposits on the surface of the printed object after drying of the ink is sometimes noted, whether drying is done by thermal radiation (IR) or UV.
These problems arise despite proper adjustment of the dry offset printing equipment.